Problem: consistency is not checked on startup

[yihuang]

if the memiavl directory is dirty, we might get errors later down the stream.

do some checks on startup to find issues early on:

• tmp snapshot directories
• snapshots that's later than the "current" one
• the "current" snapshot do exists.
• wal covers all the snapshots.

[leejw51crypto]

also print how to recover